It is well known to use the naphtha paraffin product as obtained in a Fischer-Tropsch process as steam cracker feedstock. For example, in “The Markets for Shell Middle Distillate Synthesis Products”, Presentation of Peter J. A. Tijm, Shell International Gas Ltd., Alternative Energy '95, Vancouver, Canada, May 2-4, 1995 on page 5, it is mentioned that SMDS naphtha, the Fischer-Tropsch derived naphtha fraction of the Shell MDS process, is used as steam cracker feedstock in for example Singapore.
Because a Fischer-Tropsch derived product contains almost no detectable levels of sulphur, sulphur will have to be added to this feedstock before it can be used as steam cracker feedstock. This may be done by adding a sulphur additive like dimethyl disulphide (DMDS) or by blending the Fischer-Tropsch feed with a high sulphuric content material as described in more detail in ‘Preliminary Survey on GTL Business Based on SMDS technology, June 2001, Japan External Trade Organization (JETRO), section 6.2.3.
WO-A-2003062352 discloses a process wherein lower olefins are prepared in a steam cracker furnace designed for petroleum naphtha starting from a Fischer-Tropsch derived gas oil.
US-A-2003/0135077 describes a process wherein a so called Fischer-Tropsch derived syncrude is blended with a sulphur containing petroleum derived naphtha and a treated crude derived fraction boiling above the naphtha boiling range. The petroleum derived naphtha, the Fischer-Tropsch syncrude and the refined heavy petroleum-derived portion are forwarded to a naphtha cracker unit for processing.
A problem of the process of US-A-20030135077 is that such relatively heavy and wide boiling feedstocks will foul the tubes in the convection section preheaters and/or downstream equipment of a steam cracker furnace by coke deposition.
The object of the present invention is to provide a process that can convert a relatively heavy feed comprising a Fischer-Tropsch derived synthesis product into lower olefins.